For a Long Term Evolution (LTE) system, a preceded codebook is generated in accordance with Discrete Fourier Transformation (DFT) vector, and a two-stage codebook structure is adopted. Taking an 8-antenna codebook in Release 12 (Rel-12) as an example, for a first stage codebook, a base station (evolved Node B (eNB)) determines a subset of DFT beam vectors in accordance with a Preceding Matrix Indicator (PMI) 1 fed back by a User Equipment (UE), and this PMI 1 is just an index in a set containing all the subsets of beam vectors. The UE then feeds back the PMI 1 to the base station (eNB). For a second stage codebook, the eNB performs column selection on the determined subset of beam vectors in accordance with a PMI 2 fed back by the UE, so as to select one or more columns of beam vectors therefrom and preform phase adjustment between polarization directions. Next, the eNB generates a final precoding matrix in accordance with the PMI 1 and the PMI 2 from the UE for downlink data transmission.
Along with the development of the antenna technology, an active antenna has been proposed so as to individually control each element. Through this design, the antenna array is provided with a two-dimensional (2D) structure arranged in both a horizontal direction and a vertical direction, rather than being arranged merely in the horizontal direction. This antenna array with the 2D structure is called as a three-dimensional (3D) Multiple-Input Multiple-Output (MIMO) antenna array.
There are various schemes for the design of a codebook for the 3D MIMO antenna array, and one of them lies in the extension on the basis of the 8-antenna codebook in Rel-12.
To be specific, for the first stage codebook, a subset of DFT beam vectors is acquired through a Kronecker product of a subset of vertical-dimension beam vectors and a subset of horizontal-dimension beam vectors, and for the second stage codebook, a subset of DFT beam vectors is acquired through column selection and phase adjustment on the subset of beam vectors acquired for the first stage codebook. There exist the following two column selection modes for the second stage codebook.
In a first column selection mode, multiple columns of vertical-dimension beams and multiple columns of horizontal-dimension beams are selected from a group of vertical-dimension beams and a group of horizontal-dimension beams of the subset of beam vectors acquired for the first stage codebook respectively. Next, a Kronecker product may be calculated in accordance with the selected columns of horizontal-dimension beams and the selected columns of vertical-dimension beams. Next, the phase adjustment is performed in accordance with the result. This column selection mode is equivalent to a mode where the second stage codebook also meets the Kronecker product. Each group of beams is acquired by dividing the subset of beam vectors.
In a second column selection mode, it is unnecessary to differentiate the vertical-dimension beams and the horizontal-dimension beams. Instead, each beam vector in the subset of beam vectors acquired for the first stage codebook is considered as a whole, and the phase adjustment is performed on multiple columns of beams selected from the subset of beam vectors. In the case that a Rank Indicator (RI) of the precoding matrix is greater than 1, it is impossible for the second stage codebook acquired in this column selection mode to meet a Kronecker product structure. However, each of the selected columns of beam vectors meets the Kronecker product structure, which is also called as a partial Kronecker product structure.
In the codebook for the 3D MIMO antenna array in the related art, the first stage codebook has a complete Kronecker product structure, and the number of the beams is a product of the number of the beams in the group of vertical-dimension beams and the number of the beams in the group of horizontal-dimension beams. In the case that there are a plurality of beams in the group of vertical-dimension beams and a plurality of beams in the group of horizontal-dimension beams, a relatively large product is acquired. In other words, the first stage codebook includes a large number of beams, resulting in a plurality of combinations during the column selection for the second stage codebook, i.e., a large number of codebooks based on which the second stage codebook is selected. A feedback period of the second stage codebook is short, so a feedback overhead may increase due to the large number of codebooks.